Hey, firstly, I have no scientific research on this matter, just my personal opinion and some collected info from Spanish speakers.

So I'm inclined to think that Spanish and Slavic/Baltic languages have many similarities (in vowels maybe?). Certain people from Spain actually stated that speakers from Slavic regions can imitate Spanish accent rather easily, and generally are more understandable than the other foreigners. Any Spanish speaking person can confirm this?

I myself, despite not speaking Spanish at all, and being from Baltic region, don't have much trouble with listening in Spanish language (though this might have been an effect of Spanish dramas I had been watching on TV as a child). Also, the words don't seem to be very difficult to pronounce and I feel that I can get them right (although a Spanish speaker would have yet to confirm this). Compared to French, where all the vowels and consonants are like tongue twisters to me, Spanish just seems to be much easier.

So any ideas? Is it true that Spanish (and maybe Portuguese) and Slavic/Baltic languages have lots in common phonologically?

I am a native Slavic speaker, and pronouncing Spanish is easier to me than pronouncing English. First of all, you're right that the vowel inventory of Spanish is very simple and resembles that of most Slavic languages (no independent phonemes like [æ], [ø], [y]). Secondly, there are coincidences in consonants: alveolar flap, trilled r, some palatal sounds, dental /t/ and /d/. So there's nothing about the Spanish phonology that would be hard to get for a Slavic speaker.

That said, this is just a curious fact, which does not point at any relationship between them beyond being together in the Indo-European family.

I can make a recording of my Spanish if any native speaker wants to listen to my accent.

The five-vowel system of these languages is the single most widely-attested one worldwide and an alveolar flap or trill is the most common form of rhotic. It's not that Slavic and Spanish are particularly similar, it's that languages like English and French are outliers among languages of the world.

It's odd for me to hear that the intonation is similar when (a) intonation varies significantly across Baltic and Slavic and (b) some of these systems are quite divergent. Russian, for instance, has heavy stress accent leading to the kind of vowel reduction characteristic of English. Vowel reduction is found in Spanish's close relatives Portuguese and Catalan, but not in mainstream varieties of Spanish itself. It's considered a "syllable-timed" language by those who consider this a valid distinction to make. Other varieties of Slavic have vowel length distinctions (e.g. Czech) or pitch-accent (Slovenian).

I have never overhead someone speaking a Slavic language and mistook it for Spanish or overheard Spanish and mistook it for any Slavic language. They just aren't that similar. You want to see a European language that sounds very similar to Spanish, look at Greek.

"Richmond is a real scholar; Owen just learns languages because he can't bear not to know what other people are saying."--Margaret Lattimore on her two sons

IME Spanish is relatively easy for Indians to pronounce as well. This is partly because of its five-vowel system and relatively phonemic orthography but also partly because of the dentals (or whatever they are). It almost feels as if every single sound in Mexican Spanish also exists in Malayalam. The fact that some Spanish-speakers seem to have [h] instead of [x] makes it even easier.

Maybe, but impressionistically speaking, I would say that at least to me, Spanish and most Indian languages (regardless of genetic affiliation) sound like they mostly share the same dental stops, except that Indian languages, especially Indo-Aryan ones, also have aspiration and breathy voice. Also, dental nasals appear to be somewhat more common in Malayalam and Tamil than in other Indian languages, and in Spanish, they're even more common and sound like they're practically everywhere. Meanwhile, most native speakers of English do not appear to have these dental consonants, and Portuguese, Italian, and perhaps even Romanian almost sound like they can't decide whether to use dentals or alveolars so they just use both in free variation. I've discussed this sort of thing with dEhiN before, and he seems to agree with me on some of this.

I am a native speaker of Spanish, I'm learning Russian and I have a basic level of Hindi. And I'd never had any trouble pronouncing Russian words. My teacher of Russian said to me that I have a good pronunciation, so I suppose it may be true that there's not a big phonetic difference between these languages. But that's the only similitude I can find between them

I think linguoboy is right that this is more because Spanish has certain features that are universally common than due to any particular resemblance between Spanish and Slavic languages in particular. That being said, I do have a question about this:

linguoboy wrote:The five-vowel system of these languages is the single most widely-attested one worldwide

Is that actually true? I thought the three-vowel system (/a/ vs. /i/ vs. /u/) was. But I can certainly agree that the five-vowel one is one of the most common.